The neurologic form of equine herpesvirus-1 (EHV-1, also called myeloencephalopathy) is highly contagious and multiplies within its host very rapidly, making early detection and prompt treatment paramount goals in disease control. During a presentation at the 2011 Western Veterinary Conference, held Feb. 20-24 in Las Vegas, Nev., Steve Reed, DVM, Dipl. ACVIM, an associate at Rood & Riddle Equine Hospital, in Lexington, Ky., discussed the current methods for diagnosing, treating, and preventing EHV-1.

"Outbreaks (of EHV-1) have occurred across the world but recent outbreaks in the U.S. have helped raise awareness of the disease," Reed said.

Earlier this year, a large-scale EHV-1 outbreak killed or sickened dozens of horses in the western U.S. and Canada. The outbreak was believed to have stemmed from a single horse show held in early May in Odgen, Utah.

In early 2003 another EHV-1 outbreak killed 12 horses and caused neurologic signs in 30 additional horses at the University of Findlay's English equestrian center. Of the 135 horses on the premises, 117 showed clinical signs associated with EHV-1, including fever and respiratory abnormalities.

A variable that made the outbreak even more serious was the eventual discovery that the particular strain of EHV-1 affecting the Findlay horses was a mutated one. Thus, the horses were not protected by the EHV-1 vaccine they'd received. Additionally, Reed noted that this particular mutation caused the virus to rapidly multiply within the horses' bodies and cause an unusually high fatality rate among the animals.

Findlay staff members were eventually able to gain control of the situation; however, the outbreak served as a reminder of the damage EHV-1 can cause.

Diagnosis

Reed explained that once infected with the neurologic form of EHV-1, a horse typically begins showing signs of neurologic disease within five to 10 days. Once the clinical signs become apparent, "this virus replicates very, very rapidly," he noted.

He explained that clinical signs can begin as progressive, ascending ataxia (incoordination), a unilateral (seen in one limb) front leg lameness, or hind leg weakness. Reed noted that some horses present with fever or urinary and fecal incontinence, but these are all inconsistent clinical signs. He said the telltale clinical sign is if neurologic abnormalities progress rapidly through a group of horses.

The disease can also present as respiratory deficiencies and abortions in broodmares. While horses affected by these manifestations can develop neurologic signs, it's not an overly common occurrence.

Obtaining virus isolation from a nasal swab, blood sample, or a nervous system tissue sample (which has been considered the gold standard for many years);

Using polymerase chain reaction (PCR) testing to locate the virus antigen in pharyngeal (from the pharynx) or nasal secretions;

Utilizing PCR to identify virus antigen in the blood (although negative PCR tests do not rule out EHV-1 as the cause of the disease, this test is very sensitive and might detect virus that has been there in the past or is currently present, but not causing the clinical signs in that horse. Thus, it is very sensitive but not as specific, although the sensitivity and specificity are both relatively good, Reed said.); and

He added that a positive PCR test in a horse not currently linked to an outbreak (i.e., one that hasn't been in contact with an infected horse or kept at a barn where EHV-1 has been detected) is more problematic than one in a horse believed to be exposed to the virus, as the former could indicate a latent infection.

Management and Treatment

Reed stressed that because EHV-1 spreads very rapidly, veterinarians and horse owners should isolate horses they suspect are infected as soon as possible. He also described a conundrum that has been discussed at length in the veterinary community: "If the virus is in the environment, can you get away with quarantining just one horse if the horse has been around other horses? This would be quite risky, but if the horses that have been in contact with the affected horse are watched carefully for several days looking for signs of infection one might get away with some form of limited quarantine. Each veterinarian needs to use their own knowledge and common sense as well as proper interpretation of the national, state, and local regulations."

For horses in the midst of an outbreak that were not previously exposed to EHV-1, Reed recommends vaccination: "In the face of an outbreak, for animals who haven't been exposed, I think vaccination is important to keep horse immune systems on proper surveillance for infectious diseases. Personally I'd use a modified-live vaccine (over a killed vaccine). This is my personal opinion about which I think could be most effective, but it is important to say that there are several killed vaccines which have been shown to help develop good humoral (in the blood) and some cellular immunity responses."

Although the neurologic form of EHV-1 proves fatal for many infected horses, others (typically those cases caught early) respond to treatment and recover from the infection. Reed stressed, though, that once a horse is infected, he likely carries the disease for the rest of his life.

Reed said common medications to treat clinical signs of EHV-1 include dexamethasone, flunixin meglumine (Banamine), dimethyl sulfoxide (DMSO), and valacyclovir (an antiviral drug that is now the recommended treatment). He also noted that continuous nursing and medical support are crucial for EHV-1 patients.

Additionally, he said he's hoping for some new treatment options to be developed in the relatively near future: "I think we're going to see some new antiviral meds come on the scene."

Latent Infections

A challenge that many horse owners face is that a relatively large percentage of horses might be latent EHV-1 carriers, Reed noted.

He discussed a study in which he and colleagues tested 132 broodmares in Central Kentucky and determined that 46% of them were latent carriers. More so, 8% of those mares carried a mutated strain of EHV-1 similar to that discovered at Findlay.

"This virus is in the environment," Reed cautioned. "It's out there."

Reed explained that a latent infection will cause no problems for the horse until reactivated by a trigger, such as stress. Latent carriers essentially allow the disease to carry on, eliminating the ability to eradicate the disease from the equine population. This is the known method of how herpesviruses remain in the environment, he noted.

About the Author

Erica Larson, news editor, holds a degree in journalism with an external specialty in equine science from Michigan State University in East Lansing. A Massachusetts native, she grew up in the saddle and has dabbled in a variety of disciplines including foxhunting, saddle seat, and mounted games. Currently, Erica competes in eventing with her OTTB, Dorado.

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